The present invention relates to a method of removing the acid liberated from the electrodeposition bath in cathodic electrodeposition coating while the coating film is being deposited.
In cathodic electrodeposition coating, the substrate to be coated is immersed in an aqueous electrodeposition bath and connected as the cathode. When a voltage is applied, a coating film is deposited on the substrate. The coating materials employed comprise polymers that have been converted by protonation to a water-dispersible form. This protonation is achieved predominantly through the addition of weak organic acids. These acids accumulate in the region of the anode if in the course of coating deposition the cationic binder is neutralized and the coating material that has been consumed is gradually replaced by new, protonated coating material. In order to control the pH of the electrodeposition bath, therefore, it is necessary to remove the acid from the bath. This is generally done by means of what is known as the anolyte circuit. The anolyte circuit initially requires that the anode is separated from the remainder of the electro-deposition bath by a diaphragm or membrane. This membrane is generally an anion exchange membrane, which permits only anionsxe2x80x94in the present case, acid radicalsxe2x80x94to flow toward the anode. Binders and pigments, on the other hand, are held back. By means of the anolyte circuit, acid-enriched electrolyte is withdrawn from the anode compartment, discarded generally as wastewater, and replaced by water.
In addition to the anolyte circuit, electrodeposition baths normally include an ultrafiltration circuit. This circuit removes bath liquid directly and passes it to an ultrafiltration stage whose purpose is to separate out solvents and other coating components of low molecular mass that accumulate in the bath. Binders and pigments, on the other hand, are retained and passed back to the bath. For further details regarding the prior art reference may be made to the literature (e.g. xe2x80x9cfarbe+lackxe2x80x9d, 2/1981, p. 94 ff).
For removing the acid which accumulates in the bath U.S. Pat. No. 3,682,814 proposes breaking down at least part of this acid by oxidation at the anode. Auxiliary measures are considered here for effective implementation of the oxidation, such as the heating of the anode zone or the addition of a catalyst solution. It is additionally intended that anodes by used which are coated with platinum, platinum oxide and other noble metals, chromates, manganates, vanadates, molybdates, cobalt, nickel, chromium and oxides of these metals, and other heavy metals. The acid to which U.S. Pat. No. 3,682,814 gives particular preference in its procedure is formic acid. The reason for this is that the oxidative dissociation of formic acid to carbon dioxide and water consumes two charge units per dissociated molecule. The acid in the bath can therefore be broken down electrochemically with a theoretical maximum of 50%. Other common electrocoating bath acids require more charge units per molecule for their anodic oxidation to carbon dioxide and water, and therefore have even lower theoretical maximums.
Using the process specified in U.S. Pat. No. 3,682,814 it is possible according to Example 1 therein to break down about 40%xe2x80x94out of a theoretical maximum possible 50%xe2x80x94of the formic acid neutralized at the anode. A disadvantage of this process is the instability of the anodes employed. In addition, anode and cathode are separated by a membrane, since the temperatures and pH for a particularly efficient reaction in the anode compartment are different than those prevailing in the cathode compartment.
DE-44 09 270 also proposes anodic oxidation of the acid employed. As already preferred in U.S. Pat. No. 3,682,814 the acid employed here is again essentiallyxe2x80x94in other words, to an extent of more than 90%xe2x80x94formic acid. Theoretically possible anode materials specified are platinum and platinized stainless steel electrodes, platinized titanium electrodes, platinized graphite electrodes, ruthenium-doped stainless steel electrodes or mixed-oxide-doped electrodes made from stainless steel, titanium or graphite. Unlike U.S. Pat. No. 3,682,814, however, this patent gives no measurement results and no numerical data for the breakdown rates achieved.
With regard to the electrodes that can be employed in electrochemical processes, German Patent 15 71 721 discloses electrodes that are employed inter alia as anodes for the chloralkali electrolysis. To avoid losses and improve electrode resistance, use is made of oxides of platinum, iridium, rhodium, palladium, ruthenium, manganese, lead, chromium, cobalt, iron, titanium, tantalum, zirconium or of silicon. Further fields of use of said electrodes are in electrodialysis and electrodeposition coating.
German Patent 16 71 422, furthermore, discloses the use in the alkali metal chloride electrolysis of an anode comprising a titanium core with a mixed coating covering at least part of the core surface and comprising a material formed from ruthenium oxide and titanium oxide which is resistant to the electrolytes and to the electrolysis products. These anodes exhibit a substantially lower degree of overvoltage and at the same time are dimensionally stable. Building on these properties it was possible to develop cell constructions, such as membrane cells, and hence to improve the performance of the mercury cells and diaphragm cells known hitherto.
DE-A 34 23 605 describes composite electrodes comprising an electroconductive polymer and, embedded partly therein, catalytic particles (support particles with applied catalyst) and processes for their preparation. They can be employed, for example, as an oxygen anode in the electrolytic recovery of metals from aqueous solutions. Further fields of use that are specified are electrodialysis and electrodeposition coating.
EP-B 0 296 167 likewise describes the use of comparable electrodes (referred to as dimensionally stable anodes, DSA) in cathodic electrodeposition coating. These electrodes are neither dissolved nor destroyed in the course of electrophoretic coating under the electrodeposition conditions assumed therein, i.e. in respect of coating formulation, current density, pH and the destructuve influence of chlorine.
In the field of wastewater treatment, as well, various electrodes have been tested and employed for the oxidative breakdown of substances. In the course of such tests and use it has been found, in particular, that electrodes with a coating of tin oxide (SnO2) are highly effective in the electrochemical breakdown of organic substances. These electrodes have in particular also been tested in comparison with conventional electrodes such as the abovementioned DSA electrodes, for example (Stucki, Kxc3x6tz, Carcer, Suter: xe2x80x9cElectrochemical waste water treatment using high overvoltage anodes, Part II: Ahode performance and applicationsxe2x80x9d, Journal of Applied Electrochemistry 21 (1991), 99-104; Comnimellis: xe2x80x9cTraitement des eaux rxc3xa9siduaires par voie xc3xa9lectrochimiquexe2x80x9d, gwa 11/92, 792-797; Comninellis: xe2x80x9cElectrochemical treatment of waste water containing phenolxe2x80x9d, Electrochemical Engineering and the Environment 92, Symposium Series No. 127, 189-201).
The present invention has now set itself the object of providing a method of removing the acid liberated in cathodic electrodepostion coating in the course of the deposition of the coating film which reduces the number of rinsing procedures via the anolyte circuit that are required to remove acid and possible breakdown products of the acid or which manages completely without the anolyte circuit.
This object has surprisingly been achieved by removing the liberated acid, preferably formic acid, by oxidation at anodes coated with a layer of ruthenium oxide, iridium oxide or tin oxide or with a mixture of these oxides. With the procedure of the invention is has been possible to break down, oxidation, more than 49% of the acid in the bath. This comes close to the theoretical maximum of 50% and is a considerable improvement on the levels of around 40% specified in U.S. Pat. No. 3,682,814. Such increased efficiency makes it possible to reduce the number of flushing procedures required to remove the acid via the anolyte circuit. Furthermore, the electrodes employed by the invention are more chemically stable toward the medium.
It is also possible in accordance with the invention to employ acids other than formic acid. Such acids, however, and generally less favorable owing to their lower theoretical maximum capacity for electrochemical breakdown. Lactic acid, for instance, can be broken down electrochemically only to an extent of about 35%.
Surprisingly, it is even possible to do entirely without the anolyte circuit; in other words, the electrodes of the invention can be employed directly in the electrodeposition bath and it is no longer necessary to separate the anode from the cathode compartment by a membrane. In this case, the invention employs additional methods to reduce the acid content. These methods preferably comprise conventional membrane methods. These methods preferably begin with the ultrafiltrate, since the latter is already devoid of the relatively high molecular mass constituents of the coating material. Examples of suitable membrane methods are methods operating by means of dialysis, osmosis, reverse osmosis, electrodialysis or a further downstream ultrafiltration. Methods of this kind are described, for example, in DE-44 09 270, EP-262 419, U.S. Pat. No. 4,971,672 or U.S. Pat. No. 5,091,071.
As demonstrated by Example 2 below, it is possible under these conditionsxe2x80x94i.e. without the anolyte circuitxe2x80x94to remove about 65-70% of the acid from the bath. The remaining 30-35% of acid can be expelled by means of a membrane method, e.g. electrodialysis. The use of such additional measures is of course also possible in baths operating with an anolyte circuit.
In the method of the invention the electrodeposition coating baths preferably employed are those whose binders comprise synthetic resins that have cationic groups. These binders are preferably protonated reaction products of epoxide-functional synthetic resins and amines. Protonation here preferably involves formic acid, acetic acid, lactic acid and dimethylolpropionic acid. Special preference is given to the use of formic acid. These acids are oxidized at the anode into water and carbon dioxide.
The substrate of the electrodes of the invention can consist of metal or conductive plastic. In the case of the metals it preferably comprises titanium, tantalum, niobium or an alloy of these metals. A suitable example is an alloy of titanium and from 1 to 15% by weight molybdenum. A particularly preferred substrate is titanium.
The layer of ruthenium, iridium or tin oxide, or of a mixture of these oxides, that is applied to the anodes employed in accordance with the invention preferably has a thickness of from 0.01 to 10 xcexcm. A particularly preferred range is 0.1-7 xcexcm.
The text below describes experimental examples with the method of the invention, demonstrating especially the improvements over the prior art.